The so-called "resist-heating effect" has been observed in electron-beam microlithography with a variable-shaped electron beam. See, e.g., Japanese Laid-Open Patent Document No. Hei 2-143516. The resist-heating effect causes a pattern transferred with a large diameter electron beam to be larger than the same pattern transferred with a smaller diameter electron beam even though the dose is constant. The electron beam irradiating the resist heats the resist, increasing the resist's sensitivity. This effect is most often observed in processes involving direct electron-beam writing of patterns.
In electron-beam writing using a variable-shaped electron beam, the resist-heating effect can be reduced or compensated by changing the exposure time for each of the various beam sizes to achieve a selected dose that produces a corresponding pattern dimension.
While adjusting dose is adequate to compensate the resist-heating effect in direct writing of patterns, dose adjustment is generally impossible in projection or proximity pattern transfer in which patterns are transferred from a mask. A mask pattern used to define a circuit pattern typically contains a mixture of large and small circuit features. When the mask is illuminated, both large and small pattern features are illuminated simultaneously and the dose cannot be optimally selected for both. Under such conditions, the resist-heating effect cannot be eliminated.
Accordingly, methods and apparatus for reducing or compensating for the resist heating effect in CPB pattern transfer from a mask to wafer are needed. There is also a need for such methods and apparatus that avoid the so-called "stencil" or "donut" problem.